In the past few decades our society's increasing demands for energy have naturally resulted in increased utilization of renewable resources such as solar energy. One of the most common techniques of directly tapping solar energy involves the use of photovoltaic devices such as silicon solar cells. In general, solar cells are deployed in large solar arrays including numerous solar cells which are intricately positioned and interconnected to provide optimum electricity production.
A common problem in the use of solar cells is the requirement that they be protected from the terrestrial elements over a prolonged period of time. For example, in order to be commercially successful, solar arrays must be suitably encapsulated to achieve at least a 20-year lifetime expectancy in terrestrial environments. This has resulted in a concerted effort to develop a suitable encapsulating material to protect solar arrays from the elements over a sustained period of time.
Polymer films have been a natural choice as possible solar cell encapsulants. However, it has been a most difficult technical problem to develop proper polymers for encapsulating the solar cell arrays to protect the optically and electrically active elements from the degrading effects of typical terrestrial environments. In general, solar cell encapsulants have included three layers--the pottant, an adhesive and a weather resistant layer. The layer directly surrounding the solar cell is known as the pottant. The pottant insulates and protects the delicate mechanical and electrical elements of the solar cell against vibrations resulting from wind, earthquakes and other possible external forces. The adhesive layer is necessary in order to secure the hard outer weather resistant layer to the relatively soft shock proof pottant layer. The weather resistant layer may be composed of different materials such as a hard acrylic polymer on the top and bottom surfaces and black or silicon rubber as a sealant along the edges. This layer functions to protect the solar cell from rain, dust and other debris.
Plastics are finding increasing use in construction as decorative panels, protective films, clear panels and are increasingly being used in air and land vehicles. Many of these plastics degrade due to ultraviolet and infrared components of solar radiation, wind, rain and thermal cycling they are exposed to in the terrestrial environment.
In order to evaluate plastics for suitability for long term use, aging tests have been developed. Indoor tests use artificial light which does not accurately reproduce solar radiation. Laboratory tests are absent the effect of wind and rain and do not utilize light and dark cycling as experienced in an outdoor environment. Outdoor tests in which a panel of plastic is mounted in a fixture facing the sun do subject the sample of panel to realistic environmental conditions. However, it requires very long test periods to reach induction of a change of property such as elongation failure.